1. Field of the Invention
The present invention relates to configurations of motion image decoding methods and apparatuses decoding a signal coded and compressed in a system such as the Moving Picture Experts Group (MPEG) compression.
2. Description of the Background Art
In a field for example of digital TVs, MPEG systems are known as an image coding system used to code and compress image data.
The MPEG systems are represented by MPEG-1 and MPEG-2. MPEG-1 is only intended for non-interlaced images, whereas MPEG-2 is intended not only for non-interlaced images but also interlaced images.
Such MPEG coding adopts motion compensation prediction (temporal compression), discrete cosine transform (DCT) (spatial compression), and entropy coding (variable-length coding). In the MPEG coding, timebase predictive coding is initially effected for each macroblock serving as a unit, hereinafter referred to as a “unit macroblock.” In MPEG-1 frame-based predictive coding is effected and in MPEG-2 frame- or field-based predictive coding is effected.
The macroblock is formed for example of a Y signal (a luminance signal) having a size of 16 (a number of pixels in a horizontal direction)×16 (a number of pixels in a vertical direction), a Cb signal (a color difference signal) having a size of 8 (a number of pixels in a horizontal direction)×8 (a number of pixels in a vertical direction), and a Cr signal (a color difference signal) having a size of 8 (a number of pixels in a horizontal direction)×8 (a number of pixels in a vertical direction).
Hereinafter, for the sake of convenience, only the Y signal will be described. Corresponding to a predictive coding system, there exist three types of pictures, an I picture, a P picture and a B picture. The following description will be provided in conjunction with intraframe predictive coding by way of example.
(1) I picture: a picture coded only from intraframe information. It is generated without using interframe predictive coding. In an I picture, any macroblock type is intraframe predictive coding using only intraframe information.
(2) P picture: a picture generated by effecting a prediction from an I or P picture. Typically in a P picture a macroblock type includes both intraframe coding using only intraframe information and forward interframe predictive coding providing a prediction from a previously reproduced image.
(3) B picture: a picture produced by bidirectional prediction and typically including the following macroblock types:
a) intraframe predictive coding using only intraframe information;
b) forward interframe predictive coding providing a prediction from a previously reproduced image;
c) backward interframe prediction coding providing a prediction based on future; and
d) interpolative interframe predictive coding depending on both a prediction based on past and that based on future.
The interpolative interframe prediction herein refers to averaging out the two predictions, forward and backward predictions, between corresponding pixels.
As such, if an MPEG decoder effects a decoding process for all such macroblock types it would typically require two pictures of memory for a reference image memory for storage of an I or P picture.
To address this, Japanese Patent Laying-Open No. 2000-4440 discloses a motion image decoding method and apparatus contemplating a reduced memory capacity by compressing and thus storing data to a frame memory in decoding a motion image compressed for example in MPEG.
More specifically, Japanese Patent Laying-Open No. 2000-4440 discloses a motion picture decoding method which can be implemented by comprising a first step of inversely DCTing an MPEG compressed and coded signal and using resultant image data or the resultant image data and reference image data to generate first reproduced image data, a second step of Hadamard transforming and thus coding the first reproduced image data to generate second reproduced image data having an amount of data smaller bitwise than the first reproduced image data, a third step storing to a reference image memory the second reproduced image data obtained at the second step that is required to generate reference image data, and a fourth step using the second reproduced image data in the reference image memory to generate reference image data corresponding to the first reproduced image data.
In the above method, however, if orthogonal transform is followed by a rounding operation the rounding operation is effected in a single direction. This results in an increased probability of rounding-up in a picture, as seen in its entirety, and thus can provide error accumulation resulting in noticeable degradation.